This paper reviews advances in methods for estimating fluvial transport of suspended sediment and nutrients. Research from the past four years, mostly dealing with estimating monthly and annual loads, is emphasized. However, because this topic has not appeared in previous IUGG reports, some research prior to 1990 is included.
The motivation for studying sediment transport has shifted during the past few decades. In addition to its role in filling reservoirs and channels, sediment is increasingly recognized as an important part of fluvial ecosystems and estuarine wetlands. Many groups want information about sediment transport [ Bollman, 1992]: Scientists trying to understand benthic biology and catchment hydrology; citizens and policy-makers concerned about environmental impacts (e.g. impacts of logging [ Beschta, 1978] or snow-fences [ Sturges, 1992]); government regulators considering the effectiveness of programs to protect in-stream habitat and downstream waterbodies; and resource managers seeking to restore wetlands.
Developing better methods for estimating sediment transport is important for a second reason: The methods can also be used to estimate nutrient transport. Nutrients play a critical role in determining the ecology of rivers, lakes and estuaries. Scientists, regulators, and resource managers increasingly want to understand nutrient fluxes.
Tools for estimating sediment and nutrient transport are changing. Recent research has been directed at developing methods for reducing the number of samples required to obtain acceptable load estimates. In particular, rating curve estimators and statistically-based sampling [ Thomas, 1985; Thomas, 1991; Thomas and Lewis, 1993; Thomas and Lewis, 1995], requiring fewer samples at lower cost, have attracted the most attention. A motivation for developing these methods is economics. Traditional methods for estimating sediment loads [ Porterfield, 1972] call for near-daily sampling over many years. Such sampling programs appear to be increasingly vulnerable to government cost-cutting. For example, most long-term daily sediment stations in Canada were recently discontinued in the belief that data needs could be met with lower cost alternative sampling methods [ Spitzer, 1991; Smith, 1991; Ongley, 1992; Yuzyk et al., 1992; Day, 1988; Day, 1992].
At the same time, users of data are demanding higher quality information. Results need to be objective and reproducible. The precision of results must be reported. While traditional ``worked records'' based on daily data may produce good estimates of sediment and nutrient transport, the quality of these estimates is difficult to assess [ Gilroy, 1991]. In contrast, the precision of ``rating curve'' [ Gilroy et al., 1990] and statistically-based sampling estimators [ Thomas, 1985], are readily computed.